1. Field of the Invention
This invention relates to a radiation image read-out apparatus, and more particularly to a radiation image read-out apparatus in which stimulated emission emitted from a radiation image convertor panel is detected by a line sensor formed of a CCD.
2. Description of the Related Art
When certain kinds of phosphor are exposed to radiation such as X-rays, they store a part of energy of the radiation. Then when the phosphor which has been exposed to the radiation is exposed to stimulating light such as visible light, light is emitted from the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is generally referred to as “a stimulable phosphor”. In this specification, the light emitted from the stimulable phosphor upon stimulation thereof will be referred to as “stimulated emission”. There has been known a radiation image recording and reproducing system, comprising a radiation image recording apparatus and a radiation image read-out apparatus, as a CR (computed radiography) in which a layer of the stimulable phosphor is exposed to a radiation passing through an object such as a human body to have a radiation image of the object stored on the stimulable phosphor sheet as a latent image, stimulating light such as a laser beam is projected onto the stimulable phosphor sheet, and the stimulated emission emitted from the stimulable phosphor sheet is photoelectrically detected, thereby obtaining an image signal (a radiation image signal) representing a radiation image of the object.
There has been known a radiation image convertor panel comprising a stimulable phosphor layer formed on a substrate as a recording medium employed in the radiation image recording and reproducing system. As the radiation image read-out apparatus, there has been known an apparatus which detects stimulated emission emitted from a radiation image convertor panel upon exposure to a line-like stimulating light beam extending in a main scanning direction by a line sensor comprising a CCD having a number of light receiving portions and, while moving the line sensor and the radiation image convertor panel relatively to each other in a sub-scanning direction perpendicular to the main scanning direction. See, for instance, U.S. Pat. Nos. 6,326,636, 6,521,908, and 6,605,820. The resolution in the main scanning direction of a radiation image read out by the line sensor from the radiation image convertor panel is governed by the pitch at which the light receiving portions are arranged in the main scanning direction. Whereas the resolution in the sub-scanning direction of the same is governed by the width in the sub-scanning direction perpendicular to the main scanning direction of the stimulating light beam projected onto the radiation image convertor panel.
Since the stimulated emission emitted from a radiation image convertor panel is weak, the amplification factor becomes large in order to amplify electric charges obtained by photoelectrically converting stimulated emission and to convert the electric charges into a digital image signal representing a radiation image, which results in increase of noise in the image signal. There has been a requirement for reduction of noise in the image signal, thereby improving the quality of radiation image represented by the image signal. Accordingly, to increase in S/N of the image signal by enlarging each light receiving portion in the sub-scanning direction to increase the amount of the stimulated emission received by the line sensor is being studied. Even if each light receiving portion is increased in the sub-scanning direction, the resolution can be held unchanged in the main scanning direction and the sub-scanning direction for the reason described above.
However, this approach is disadvantageous in that when the amount of the stimulated emission received by the line sensor is increased by enlarging each light receiving portion in the sub-scanning direction, the electric potential gradation per unit length on the light receiving face is reduced and storing of the next electric charges is started before a part of the electric charges is left unreleased from the light receiving portion. The part of the electric charges left in the light receiving portion generates noise in the image signal.
Further, when the amount of the stimulated emission received by the line sensor is increased by increasing the number of rows of the light receiving portions in the sub-scanning direction with the size of each light receiving portion in the sub-scanning direction held unchanged, the image signal components obtained from respective light receiving portions adjacent to each other in the sub-scanning direction must be added into an image signal component for a pixel corresponding to the light receiving portions after respectively amplified. When adding the two image signal components after amplification, amplified noises for the two image signal components are also added. Accordingly, the ratio of the amplified noise in an image signal obtained is substantially the same as that in an image signal obtained through a row of light receiving portions, which shows that the S/N cannot be improved even if the amount of the stimulated emission received by the line sensor is increased by increasing the number of rows of the light receiving portions in the sub-scanning direction.